1. Field of the Invention
The present invention relates to a method for controlling a liquid crystal display device which prevents deterioration of the liquid crystal by quickly removing the charge stored in each liquid crystal layer after, for example, turning off of the power supply, a device for driving the liquid crystal display device, the liquid crystal display device, and an electronic apparatus incorporating the liquid crystal display device.
2. Description of the Related Art
In general, an active matrix type liquid crystal display device mainly consists of an element array substrate having a switching element provided on each of a plurality of pixel electrodes arranged in a matrix, an opposite substrate having a color filter or the like formed thereon, and a liquid crystal filling the space between the two substrates. In this structure, a liquid crystal layer is formed by each pixel electrode, the opposite substrate and the liquid crystal filling the space therebetween.
In the structure described above, applying an ON (selection state) signal onto a switching element leads to a conductive condition of that switching element. As a result, a predetermined amount of charge is stored in the liquid crystal layer connected to that switching element. Even when the switching element is brought into an OFF state by applying an OFF (non-selection state) signal after storage of the charge, stored charge in the liquid crystal layer is maintained, provided that the liquid crystal layer has a sufficiently high resistance. When the amount of stored charge is controlled by driving the individual switching elements, a change occurs in the alignment state of the liquid crystal for each pixel, thus permitting display of a predetermined information. At this point, because charge needs to be stored for each liquid crystal layer only during a portion of the period, it is possible to achieve multiplex driving using in common with the scanning lines and the data lines for the plurality of pixels by selecting individual scanning lines in a time-sharing manner.
Applicable switching elements are broadly classified into three-terminal type switching elements, such as a thin-film transistor (TFT: Thin Film Transistor) or an MOS type transistor, and two-terminal type switching elements, such as a thin-film diode (TFD: Thin Film Diode) having a non-linear characteristic. These three-terminal type and two-terminal type switching elements, having a non-linear current-voltage characteristic, are referred to also as non-linear elements.
According to the structure in which a supply of driving signal is discontinued simultaneously with the turning-off of the power supply, when turning off the power supply for a liquid crystal display device, the electric field, which has been applied to the liquid crystal layer at the moment of the stoppage of the driving signal, remains as it is, and the liquid crystal layer turns into a state that a DC voltage is applied thereto. If a DC voltage is continuously applied onto the liquid crystal layer in this state, material properties of the liquid crystal vary, leading to deterioration such as a reduced resistivity, and hence to a reduced service life of the liquid crystal display device. Therefore, it is desirable to adopt a structure in which at the time of the turning-off of the power supply for a liquid crystal display device, the supply of driving signals is continued until the charge stored in the liquid crystal layer reaches a null level.
However, because the discharge time constant of the stored charge is dependent upon various factors such as resistance and size of the pixel electrode, the material for the liquid crystal and the distance between the substrates, there is a problem that the period of time required for the charge stored in the liquid crystal layer to reach null varies for every different pixel and every liquid crystal display device. This means that the period over which the driving signals must be maintained after turning off the power supply is inconsistent and, hence, leads to a secondary problem of difficulty encountered in designing a driving signal supply circuit.
In view of the foregoing, an object of the present invention is to provide a method for controlling a liquid crystal display device, wherein liquid crystal layers are quickly cleared of any residual charge without dependency of the clearing time on individual devices, thereby suppressing degradation of the liquid crystal. The invention also is aimed at providing a device for driving the liquid crystal display device, the liquid crystal display device, and an apparatus incorporating the liquid crystal display device.
With a view in achieving the aforementioned object, the first aspect of the present invention provides a method for controlling a liquid crystal display device of the type in which desired images are displayed through control of charge amounts in liquid crystal layers of the liquid crystal display device, the method which may consist of detecting turning-off of a power supply connected to the liquid crystal display device; and upon detection of the turning-off of the power supply, electrically connecting the liquid crystal layer to a fixed potential.
According to this control method, the liquid crystal layer is connected to a fixed potential such as a grounding potential upon detecting the turning-off of the power supply. The liquid crystal layers are thus quickly cleared of the charges at a constant rate. As a result, a DC voltage is never applied on the liquid crystal for a long period of time, thus permitting prevention of deterioration of the liquid crystal. It is also possible to set a period of time ending when the charge stored in the liquid crystal layer becomes null without depending upon factors such as the resistance and size of the pixel electrode of the liquid crystal display panel, the material for the liquid crystal, and the distance between the substrates and so on.
Further, in the aforementioned controlling method of the liquid crystal display device, it is desirable to electrically connect the signal line, which applies a voltage onto the liquid crystal layer, to the foregoing fixed potential upon detecting the turning-off of the power supply. Removal of charges from the liquid crystal layers can indirectly be achieved even by such a simple measure as to connect the signal lines to the fixed potential.
Further, in the aforementioned controlling method of the liquid crystal display device, it is desirable to electrically connect the signal line, which is electrically connected to the liquid crystal layer, to a predetermined voltage supplying line, and to connect the predetermined voltage supplying line to the fixed potential, upon supplying the turning-off of the power supply. Removal of charges from the liquid crystal layers can indirectly be achieved by means of simple arrangement and control, using a switch that first connects the predetermined voltage supplying line to the signal lines which supply a voltage to the liquid crystal layers and then connects the predetermined voltage supplying line to the fixed potential.
In the aforementioned method of controlling the liquid crystal display device, the arrangement is preferably such that the predetermined voltage supplying line includes a first voltage supplying line for supplying a positive voltage relative to the fixed potential and a second voltage supplying line for supplying a negative voltage relative to the same, and that the signal lines are alternately connected to the first and second voltage supplying lines. Since there are thus provided the supplying lines that supplies the voltages that are positive and negative relative to the fixed potential, and these two supplying lines are alternately connected to the signal line and at the same time these two lines are connected to the fixed potential, it is possible to remove the charge from the liquid crystal layer as the positive and negative potentials of the supplying lines converge from positive and negative potentials toward the fixed potential. It is therefore possible to easily remove the charge irrespective of whether the liquid crystal layer stores positive charge or negative charge.
Further, in the controlling method of the aforementioned liquid crystal display device, the signal line should preferably be alternately connected to the first voltage supplying line and the second voltage supplying line in response to a clock signal having a period not longer than a xc2xd horizontal scanning period. Since connection of the supplying lines and signal line is switched over in response to the high-frequency clock, the stored charge of the liquid crystal layer can be discharged rapidly irrespective of the level of the stored charge in the liquid crystal layer.
Besides, the second aspect of the present invention provides a driving device for driving a liquid crystal display device that displays a desired image by controlling an amount of charge stored in a liquid crystal layer, including: a detecting circuit that detects the turning-off of a power supply; and connecting circuit that, upon detection of the turning-off of the power supply by the detecting circuit, connects the liquid crystal layer to a fixed potential.
According to this driving device, as in the first aspect of the invention, the liquid crystal layer is connected to the fixed potential, upon detection of turning-off of the power supply. The charge stored in the liquid crystal layer is thus cleared quickly at a constant rate. As a result, it is possible to set a period of time ending when the charge stored in the liquid crystal layer becomes null without depending upon such factors as the resistance and size of the electrodes of the liquid crystal display panel, the material for the liquid crystal, and the distance between the substrates.
This driving device should preferably have a structure in which the driving device further has a first connecting circuit that connects the liquid crystal layer to a predetermined line, and a second connecting circuit that connects the predetermined line to the fixed potential, upon detection of the turning-off of the power supply by the detecting circuit. As compared with the conventional structure in which predetermined scanning signals are supplied by switching over a plurality of lines, the structure according to the present invention suffices to add only a few elements.
The driving device should preferably have a structure in which the detecting circuit detects a source voltage lower than a threshold value as indicative of the turning-off of the power supply. In order to detect the turning-off of the power supply, an arrangement which monitors the source voltage can be employed most reliably.
The driving device of the liquid crystal display device according to the second aspect of the invention should preferably have a structure in which the connecting circuit is a switching circuit that connects the liquid crystal layer with a grounding conductor when the turning-off of the power supply is detected by the detecting circuit. This is the simplest structure.
Further, according to the driving device of the liquid crystal display device, the connecting circuit should preferably electrically connect the signal line that applies a voltage onto the liquid crystal layer, to the fixed potential. It is thus possible to remove the charge from the liquid crystal layer indirectly through a simple control, for example, connecting the signal line to the fixed potential.
Further, according to the driving device of the liquid crystal display device, the connecting circuit should preferably electrically connect the signal line that is electrically connected to the liquid crystal layer, to a predetermined line, and further connect the predetermined line to the fixed potential. Removal of charges from the liquid crystal layers can indirectly be achieved by means of simple arrangement and control, using a switch that first connects the predetermined voltage supplying line to the signal lines which supply a voltage to the liquid crystal layers and then connects the predetermined voltage supplying line to the fixed potential.
In the aforementioned method of controlling the liquid crystal display device, the arrangement is preferably such that the predetermined voltage supplying line includes a first voltage supplying line for supplying a positive voltage relative to the fixed potential and a second voltage supplying line for supplying a negative voltage relative to the same, and that the signal lines are alternately connected to the first and second voltage supplying lines. Since, there are thus provided the supplying lines supplying the voltages that are positive and negative voltage relative to the fixed potential, and these two supplying lines are alternately connected to the signal line, and at the same time these two supplying lines are connected to the fixed potential, the charge can be removed from the liquid crystal layer accordingly as the supplying lines converge from positive and negative potentials towards the fixed potential. It is thus possible to easily remove the charge irrespective of whether the stored charge of the liquid crystal layer is positive or negative.
Further, in the above-mentioned driving device of the liquid crystal display device, the signal line should preferably be alternately connected to the first voltage supplying line and the second voltage supplying line in response to a clock signal having a period not longer than a xc2xd horizontal scanning period. Since connection of the supplying lines and signal line is switched over in response to the high-frequency clock, the stored charge of the liquid crystal layer can rapidly be discharged irrespective of the level of the stored charge in the liquid crystal layer.
The liquid crystal display device according to the third aspect of the present invention is a liquid crystal display device that displays a desired image by controlling an amount of charge stored in a liquid crystal layer using a scanning signal and a data signal, the liquid crystal display device including: a detecting circuit that detects the turning-off of a power supply; a control circuit that controls connections to a predetermined line upon detection of the turning-off of the power supply by the detecting circuit; a first connecting circuit that, based on the instruction from the control circuit, connects one or both of a scanning line that receives the supplied scanning signal and a data line that receives the supplied data signal, to the predetermined line; and a second connecting circuit that, upon detection of the turning-off of the power supply by the detecting circuit, connects the predetermined line to a fixed potential.
According to this liquid crystal display device, as in first aspect of the invention, the liquid crystal layer is connected to the fixed potential upon detection of the turning-off of the power supply. The charge stored in the liquid crystal layer is thus cleared quickly and at a constant speed. As a result, it is possible to set a period of time ending when the charge stored in the liquid crystal layer becomes null without depending upon such factors as the resistance and size of the electrode of the liquid crystal display panel, the material for the liquid crystal, and the distance between the substrates and so on.
The liquid crystal display device according to the third aspect of the invention includes a liquid crystal display panel having one substrate provided with a data line thereon, another substrate provided with a scanning line thereon, and a plurality of pixels each having a series connection of a non-linear element and a liquid crystal layer between the data line and the scanning line; a detecting circuit detecting turning-off of a power supply; and a switching circuit that connects a supplying line of a selection voltage applied onto the scanning line to a grounding conductor upon detection of the turning-off of the power supply by the detecting circuit.
According to this liquid crystal display device, the supplying line having a selection voltage that is applied onto the scanning line upon writing a data signal on pixels is connected to a grounding conductor, upon detection of the turning-off of the power supply. Thus, the charge stored in the liquid crystal layer is discharged quickly and at a constant rate. Particularly, the selection voltage is a voltage that turns a two-terminal type non-linear element on. It is therefore possible to remove the charge from the liquid crystal layer by turning on the non-linear element without causing a decrease in the selection voltage, immediately after the detection of the turning-off of the power supply. As a result, it is possible to set a period of time ending when the charge stored in the liquid crystal layer becomes null, without depending upon such factors as the resistance and size of the pixel electrode, the material for the liquid crystal, and the distance between the substrates.
Further, in the aforementioned liquid crystal display device, the switching circuit should preferably connect the scanning line to the supplying line supplying a voltage for turning on the non-linear element, and connect the supplying line to a grounding conductor upon detection of a turning-off of the power supply. Removal of charges from the liquid crystal layers can indirectly be achieved by means of simple arrangement and control, using a switch that first connects the predetermined voltage supplying line to the signal lines which supply a voltage to the liquid crystal layers and then connects the predetermined voltage supplying line to the fixed potential.
Further, in the above-described liquid crystal display device, the supplying line should preferably comprise a first supplying line for supplying a positive selection voltage relative to the grounding potential and a second supplying line for supplying a negative selection voltage relative to the grounding potential. The scanning line should preferably be alternately connected to the first supplying line and the second supplying line. Because there are two supplying lines employed for positive and negative voltages, respectively, relative to the grounding potential, these two supplying lines are alternately connected to the signal line, and at the same time these two supplying lines are connected to the grounding potential, the charge of the liquid crystal layer can be removed as the supplying lines converge from positive and negative potentials into the grounding potential. It is therefore possible to easily remove the charge irrespective of whether the charge of the liquid crystal layer is positive or negative.
In the liquid crystal display device according to the third aspect of the invention, the non-linear element should preferably be a two-terminal type non-linear element, and further, the two-terminal type non-linear element should preferably be a thin film diode (TFD) element having a first metal, an insulator, and a second metal.
This structure is preferred because a short circuit defect between wiring lines does not occur in principle in a two-terminal type non-linear element such as the TFD element because of the absence of a crossing portion of the wirings, and further, the film forming step and the photolithographic step can be shortened.
The liquid crystal display device of the invention may also consist of a liquid crystal display panel having a liquid crystal layer sandwiched between a substrate provided with a data line and another substrate provided with a scanning line, a detecting circuit detecting a turning-off of the power supply, and a switch circuit connecting the voltage supplying line, which applies a voltage onto the scanning line or the data line, to a prescribed constant potential upon detection of a turning-off of the power supply by the detecting circuit.
According to the passive type liquid crystal display device in which the electric field applied to the liquid crystal layer is controlled only by a pair of opposite electrodes sandwiching the liquid crystal layer therebetween, and the pixels do not have non-linear elements, the supplying line having supplied a voltage to the scanning line or to the data line is connected to a prescribed constant potential upon detection of a power supply turning-off. As a result, the charge stored in the liquid crystal layer is rapidly removed at a certain rate directly through the scanning line or the data line. It is therefore possible to determine the period of time ending when the stored charge of the liquid crystal layer reach a null level without depending upon such factors as the resistance and size of the electrode, the material of the liquid crystal, and the distance between the substrates.
Further, in the aforementioned liquid crystal display device, the scanning line or the data line should preferably be connected, upon detection of the turning-off of the power supply, to the first supplying line for supplying a positive voltage to the prescribed constant potential and to the second supplying line for supplying a negative voltage, alternately. Further, the switching circuit should preferably connect the first supplying line and the second supplying line to a predetermined constant potential. Since, there are thus provided the supplying lines supplying the voltages that are positive and negative voltage relative to the fixed potential, these two supplying lines are alternately connected to the signal line, and at the same time these two supplying lines are connected to the constant potential, the charge stored in the liquid crystal layer can be removed as the supplying lines converge from positive and negative potentials toward the constant potential. It is therefore possible to easily remove the charge irrespective of whether the charge stored in the liquid crystal layer is positive or negative.
Besides, the fourth aspect of the present invention provides an electronic apparatus incorporating the above-described liquid crystal display device, for example, a car navigation system, a portable information terminal device and various other electronic apparatuses.